1. Field of the Invention
The present invention relates to a radiation image pickup apparatus and a control method thereof, each suitable for being used for medical diagnosis, industrial nondestructive inspection and the like. In addition, it is noted that the term “electromagnetic waves” as used herein is intended to encompass X-rays, γ-rays and the like, and rays of α or β particles as well.
2. Description of the Related Art
Recently, a radiation image pickup apparatus using a non-single crystal thin film semiconductor such as amorphous silicon or the like as a solid state image pickup device to be able to radiograph a still image has been put to practical use. Enlargement in area exceeding 40 cm square, which can cover the size of the chest of the human body, has been realized using a producing technique of amorphous silicon thin film semiconductors. Because the manufacturing process thereof is comparatively easy, it is expected that an inexpensive detection apparatus will be provided in the future. Moreover, because amorphous silicon can be produced to be a thin glass plate having a thickness of 1 mm or less, amorphous silicon has an advantage capable of being produced as a detector having a very small thickness. Moreover, it is possible to implement radiographing a still image and a moving image with one apparatus, and thus unnecessary to prepare two separate dedicated apparatus for those two applications.
Such a radiation image pickup apparatus includes a photoelectric conversion circuit equipped with a plurality of conversion elements converting radiation such as X-rays into an electric signal and switching elements arranged in a matrix, and a reading circuit for reading the electric signal from the photoelectric conversion circuit. Here, the conversion elements may be made of a material that converts radiation into an electric signal directly, or may be composed of wavelength converters each converting radiation into visible light and photoelectric conversion elements each converting the converted visible light into an electric signal.
When an image is radiographed by a radiation image pickup apparatus including solid state image pickup devices using amorphous Si, an actually read image includes offsets generated by the photoelectric conversion circuit and the reading circuit. Accordingly, it is necessary to remove the offset components included in the signal value.